The present invention relates to the design of a broadband dichroic surface and a method making such a surface.
The design of microwave antennas for use in congested environments such as in the top of the stabilizer of an aircraft, places stringent limitations on the design of a rotating antenna. A dichroic reflector is one element of such an antenna structure. In such reflective antenna structures it is possible to design antenna reflectors, that will pass signals of one frequency band and reflect signals of other frequencies. The reflectors are called dichroic or frequency sensitive surfaces (FSS). The reflectors are normally made up of a large array of resonant elements, known as resonators. These resonators may be dipoles, of various configurations. The dipoles reflect certain frequencies while the dichroic surface also transmits the other frequencies. Based on the relative size of the dipoles, in relation to the dichroic surface, the reflection and transmission of frequencies is altered. Proper sizing of the dipoles will then determine the frequencies reflected while all other frequencies are transmitted.
The resonators are grouped into a grid formation on an antenna reflector to form a frequency-sensitive surface. The spacing between the resonator elements is an important design constraint in differentiating the reflected and transmitted bands.
The design and fabrication of such dichroic surfaces can be labour intensive and typically requires expensive equipment.
In known dichroic reflector techniques, conventional dichroic surfaces use blunt dipole crosses or tripole type elements that suffer from relatively narrow bandwidth. These elements produce a relatively narrow-band reflective surface due to the narrow resonances of the individual elements in the grid. The narrow resonance is due to the use of blunt ends on the elements that introduce discontinuities in the dichroic surface grid product and limit the corresponding packing density narrow resonance lines. Such surfaces are fabricated using vacuum forming, folded sheet or laser etching techniques. A number of patents disclose techniques for forming a dichroic surface implemented on an antenna reflector.
In U.S. Pat. No. 4,307,404, Young discloses a dichroic antenna system which uses a movable dichroic surface to scan multiple frequencies. Using at least one feed horn and a planar dichroic surface which can rotate about an axis, the differing phase shifts can be taken advantage of to scan selected frequencies. The method of constructing the dichroic surface is a conventional method of plating, masking, and etching of a metallic layer such as copper. Young uses thin rectangles on the dichroic surface as resonator elements.
In another U.S. Pat. No. 4,701,765, Arduini et al., discloses a multilayered dichroic antenna. Arduini shows two dichroic grids separated by a dielectric layer. The structure disclosed only reflects a single specific band of the electromagnetic spectrum. They disclose using a photoetching process of metallic layers deposited on dielectric layers.
In U.S. Pat. No. 4,814,785, Wu discloses a gridded square configuration for a dichroic surface. Wu also discloses the use of crosses, however, they are Jerusalem crosses (crosses with their arms terminated by a perpendicular bar). Regarding the fabrication method, the conventional method of etching on a substrate is disclosed. Due to the nature of the structure of the Jerusalem cross, there is a lower density of resonator elements per-unit area. Consequently, there is a maximum limit as to the amount of area covered by. the frequency selective surface and the resulting bandwidth of the surface is relatively narrow.
U.S. Pat. No. 4,835,087, Bielli et al., discloses a method of producing a dichroic surface using what may be termed as essentially conventional techniques. Bielli et al. use a computer controlled photographic projector to outline the resonator elements required on a substrate. However, to deposit the metallic conductive parts on the substrate, Bielli et al. use conventional chemical etching.
U.S. Pat. No. 4,897,151, Killackey et al. discloses a method of constructing a dichroic surface by essentially forming the metallic layer on a mandrel and then transferring the metallic layer to the final substrate. The main point of this patent is the use of a transfer technique whereby the layer is first formed on the mandrel and then transferred to substrate by having the adhesion between the metallic layer and the substrate be stronger than the adhesion between the layer and the mandrel. After the metallic layer is deposited, conventional photoresist imaging and chemical etching techniques form the grid pattern on the layer.
In contrast to the prior art cited above, the dichroic surface of the present invention has pointed resonator cross dipoles. The crosses are interlaced into a grid pattern to form a dichroic surface. The pointed ends allow the crosses to be packed with very high density and numerical analysis as well as testing indicate that this yields improved reflection bandwidth. The curvature of the antenna reflector further complicates the use of conventional techniques. The dichroic surface may be fabricated as a self-adhesive decal, conforming the dichroic surface to the surface of the reflector antenna. The decal eliminates using techniques, such as photoresist imaging and chemical etching, directly on the antenna structure.
The present invention provides a low cost dichroic surface with a pointed resonator cross grid pattern that offers enhanced bandwidth and a sharper response between frequency bands. In addition, the dichroic surface is used for broadband communications, where multiple bands of voice and data are being reflected and transmitted.
In one aspect, the invention provides a dichroic surface including a grid pattern of interlaced resonator elements, each resonator element being a resonator cross, each resonator cross having four equal length cross arms tapering to a point at each extremity of each cross arm, totalling four points per resonator cross, wherein each resonator possesses 90 degree rotational symmetry, each the form of a Greek pointed cross, and wherein the length of each cross arm forming the resonator cross, is one fourth a wavelength of the band to be reflected.
In a second aspect, the invention provides a dichroic surface deposited onto a supporting structure, which comprises a decal layer, having resonator elements, the decal layer is made of an electrically conductive metal, an adhesive layer and a decal backing. The decal layer adheres to a supporting structure using an adhesive layer; and the decal backing is discarded prior to adhering the resonator elements to the supporting structure.